This invention relates generally to valves used in fuel cell systems, and more particularly to valves used in moisture-prone environments in such fuel cell systems such that the valves do not become blocked due to ice buildup under freezing conditions, as well as methods of fuel cell system start-up under frozen conditions such that blockage due to ice formation is inhibited.
A significant benefit to fuel cells as an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, they have several environmental advantages over internal combustion engines (ICEs) and related power-generating sources. In a typical fuel cell—such as a proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell—a pair of catalyzed electrodes are separated by an ion-transmissive medium (such as Nafion™). An electrochemical reaction occurs when a gaseous reducing agent (such as hydrogen, H2) is introduced to and ionized at the anode and then made to pass through the ion-transmissive medium such that it combines with a gaseous oxidizing agent (such as oxygen, O2) that has been introduced through the other electrode (the cathode); this combination of reactants form water as a benign byproduct. The electrons that were liberated in the ionization of the hydrogen proceed in the form of direct current (DC) electricity to the cathode via external circuit that typically includes a motor or related load where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells to form a fuel cell stack or related assembly that makes up a fuel cell system.
Various fuel cell system operating conditions can lead to high water content in one or both of the reactant streams. In one form, such conditions may arise out of the use of devices such as a water vapor transfer (WVT) unit that helps ensure adequate humidity levels within various parts of the fuel cell stack. In certain operating conditions (including those associated with WVT usage), it is desirable to remove excess moisture to ensure that ice blockage of key flowpaths is avoided in conditions where such moisture may be exposed to freezing temperatures. Avoiding ice blockage is especially important during vehicle starting, where access to electricity for use in ancillary vehicular systems (such as heating, cooling, lighting and other systems) is generally not available until the fuel cell stack is operational.
Valves—with their relative movement between adjacent surfaces as a way to provide selective flow—are particularly susceptible to ice blockage, especially between such surfaces that come into intermittent contact with one another during valve opening and closing. One example are check valves, which are frequently used in fuel cell systems to limit reactant backflow into the stack during periods of non-operation of the stack in order to minimize undesirable reactions between catalytic substrates within the stack and an oxygen-bearing or hydrogen-bearing fluid. The type of complete valve closure that is needed to avoid the aforementioned reactions is often difficult to achieve, especially in situations where ice bonds are formed on valve sealing interfaces after a cold soak in a humid environment. Conventional valves (which in one form may be formed as a diaphragm that is responsive to a pressurized reactant bearing against it) exist in a deformed state at temperature for the duration of stack operation; this in turn can lead to warpage or related sustained permanent deformation that exacerbates the sealing or leakage problems. Moreover, stresses imparted to the diaphragm from the reactant is nearly uniform around the diaphragm perimeter; such relatively larger surface contact requires a significantly high reactant force, which in turn delays the onset of ice breakup and the subsequent opening of the valve. In some circumstance, this force may not be sufficient to overcome the built-up ice, leading to the aforementioned failed start.